1. Field of the Invention
The present invention relates to rare-earth alloys and a method of manufacturing such alloys. The invention also relates to Sm2Co17-based sintered magnets and a method of manufacturing such magnets.
2. Prior Art
The sintered magnet materials used in Sm2Co17-based permanent magnets are typically produced by a process which includes milling an alloy ingot of a regulated composition to a particle size of 1 to 10 xcexcm, pressing and shaping the resulting powder in a magnetic field to form a powder compact, sintering the powder compact in an argon atmosphere at 1100 to 1300xc2x0 C., and typically about 1200xc2x0 C., for a period of 1 to 5 hours, then solution-treating the sintered compact. Next, the solution-treated compact is generally subjected to aging treatment in which it is held at a temperature of 700 to 900xc2x0 C., and typically about 800xc2x0 C., for about 10 hours, then gradually cooled to 400xc2x0 C. or less at a rate of xe2x88x921.0xc2x0 C./min. In a conventional process of this type, sintering and solution treatment must be carried out under strict temperature control within an optimal range of xc2x13xc2x0 C. about the temperature setting. The reason is that, during sintering and solution treatment, the presence of a plurality of different constituent phases gives rise to local heat treatment temperature-sensitive variations in crystal grain growth and phase transitions. Moreover, temperature control during sintering and solution treatment tends to become even more rigorous for Sm2Co17-based sintered magnets of higher magnetic properties. A uniform alloy structure that is as free of segregation as possible is essential for maintaining the treatment temperature with the optimal temperature range and achieving good magnetic properties.
One casting technique used to obtain Sm2Co17-based magnet alloys having a uniform structure involves casting an alloy melt into a mold having a box-like or other suitable shape so as to form a macroscopic structure composed of columnar crystals. In such a process, the cooling rate of the alloy melt must be increased to some degree in order to form columnar crystals. Yet, in a casting process carried out using a box-shaped mold, the inner portions of the ingot tend to cool more slowly than the cooling rate at which columnar crystals form, resulting in a larger grain size and the formation of equiaxed crystals. One way to overcome this problem is to reduce the thickness of the ingot, but doing so lowers the production efficiency. Hence, ingots having a substantial degree of thickness are generally produced, often resulting in a coarser structure and the formation of equiaxed crystals. Coarsening of the structure and equiaxed crystal formation leads to segregation within the ingot, which adversely impacts the magnet structure following sintering and solution treatment, making it difficult to achieve good magnetic properties.
One solution that has been proposed is a single-roll strip casting process (JP-A 8-260083). Ingots produced by this process have a fine crystal structure and a uniform alloy structure free of segregation. However, it has been shown that sintered magnets produced from ingots with a microcrystalline structure as the starting material, while having a better coercivity than sintered magnets made from ingots cast in a box-shaped mold, have an inferior residual flux density and maximum energy product (JP-A 9-111383). Ingots with a microcrystalline structure have a much smaller average crystal grain size than ingots cast in a box-shaped mold. When these respective types of ingots are each milled into fine powders having an average particle size of 5 xcexcm during sintered magnet production, the average crystal grain size and the average particle size of the fine powder obtained by milling are similar for those ingots having a microcrystalline structure. Hence, the milled particles are not all single crystals; a greater proportion are polycrystalline, which lowers the degree of orientation when the powder is compacted in a magnetic field. The sintered magnet obtained after heat treatment thus has a lower degree of orientation, and ultimately a lower residual flux density and maximum energy product. For this reason, strip-cast ingots are not used as the starting material in the production of Sm2Co17-based sintered magnets.
Regardless of whether an ingot cast in a box-shaped mold or an ingot made by a strip casting process is used, the constituent phases of the Sm2Co17-based permanent magnet alloy after it has been cast are the same, and include a Th2Zn17 phase, a Th2Ni17 phase, a 1:7 phase, a 1:5 phase, a 2:7 phase and a 1:3 phase. Strict temperature control is required, with the optimal temperature range during sintering and solution treatment being xc2x13xc2x0 C.
It is therefore an object of the present invention to provide a rare-earth alloy which can be uniformly treated in a short period of time when heat treated as a thin strip-like ingot. It is also an object of the invention to provide a method of manufacturing such alloys.
Another object of the invention is to provide a rare-earth sintered magnet having excellent magnetic properties. An additional object of the invention is to provide a method of manufacturing such magnets.
A further object is to provide a rare-earth sintered magnet having a broad optimal temperature range for sintering and solution treatment, thereby making it possible to ease the heat temperature conditions, and in turn improving productivity. A still further object is to provide a method of manufacturing such magnets.
We have extensively studied the relationship between the alloy structure in Sm2Co17-based alloys and the structural changes that take place in such alloys when heat treated. As a result, We have found that heat treatment can be completed in a short time and a uniform structure easily achieved by the use of a Sm2Co17-based alloy ingot having a content of 1 to 200 xcexcm size equiaxed crystal grains of at least 20 vol % and a thickness of 0.05 to 3 mm.
We have also found that when such an alloy is heat-treated in a non-oxidizing atmosphere to increase the average crystal grain size, a sintered magnet can be produced which has better magnetic properties than sintered magnets produced from prior-art cast ingots.
Another discovery I have made is that a sintered magnet endowed with better magnetic properties than sintered magnets made from prior-art cast ingots can be produced by heat-treating a Sm2Co17-based magnet alloy having a fine-grained structure, that is, a Sm2Co17-based magnet alloy obtained by a strip casting process, under optimal conditions in a non-oxidizing atmosphere to increase the average crystal grain size.
In addition, I have extensively studied the relationship between alloy structure and magnetic properties in Sm2Co17-based sintered magnets, as a result of which I have discovered that by having a TbCu7-type crystal structure (referred to hereinafter as a xe2x80x9c1:7 phasexe2x80x9d) account for at least 50 vol % of the constituent phases in the starting ingot used in Sm2Co17-based sintered magnet production, better magnetic properties can be achieved than when sintered magnets are produced using prior-art cast ingots, or even when other constituent phases are allowed to serve as the major phase. This is because the 1:7 phase in a Sm2Co17-based magnet alloy has a better orientability during molding of the alloy in a magnetic field than do the other constituent phases (such as the 2:17 phase, 1:5 phase, 2:7 phase and 1:3 phase); indeed, the higher the proportion of 1:7 phase in the Sm2Co17-based magnet alloy, the better the magnetic properties that can be achieved. Furthermore, by having the 1:7 phase account for at least 50 vol % of the constituent phases, when sintering and solution treatment are carried out, local heat treatment temperature-sensitive variations do not arise in crystal grain growth and phase transitions. This allows some easing of the optimal temperature conditions for heat treatment, which until now have had to be strictly controlled.
Accordingly, in a first aspect, the invention provides a rare-earth alloy ingot made by melting an alloy composed mainly of 20 to 30 wt % of a rare-earth component R is samarium alone or at least 50 wt % samarium in combination with at least one other rare-earth element, 10 to 45 wt % of iron, 1 to 10 wt % of copper and 0.5 to 5 wt % of zirconium, with the balance being cobalt; and quenching the molten alloy in a strip casting process. The ingot has a content of 1 to 200 xcexcm size equiaxed crystal grains of at least 20 vol %, and a thickness of 0.05 to 3 mm.
In a second aspect, the invention provides a method of manufacturing rare-earth alloy ingots, which method includes the steps of melting an alloy composed mainly of 20 to 30 wt % of a rare-earth component R which is samarium alone or at least 50 wt % samarium in combination with at least one other rare-earth element, 10 to 45 wt % of iron, 1 to 10 wt % of copper and 0.5 to 5 wt % of zirconium, with the balance being cobalt; and strip-casting the molten alloy at a melt temperature of 1250 to 1600xc2x0 C. The ingot has a content of 1 to 200 xcexcm size equiaxed crystal grains of at least 20 vol %, and a thickness of 0.05 to 3 mm.
In a third aspect, the invention provides a method of manufacturing rare-earth sintered magnets, which method includes the steps of melting an alloy composed mainly of 20 to 30 wt % of a rare-earth component R which is samarium alone or at least 50 wt % samarium in combination with at least one other rare-earth element, 10 to 45 wt % of iron, 1 to 10 wt % of copper and 0.5 to 5 wt % of zirconium, with the balance being cobalt; quenching the molten alloy by a strip casting process so as to form a rare-earth alloy ingot which has a content of 1 to 200 xcexcm size equiaxed crystal grains of at least 20 vol % and a thickness of 0.05 to 3 mm; heat-treating the ingot in a non-oxidizing atmosphere at 1000 to 1300xc2x0 C. for 0.5 to 20 hours to form a rare-earth magnet alloy; milling the rare-earth magnet alloy; compression-molding the milled alloy in a magnetic field to form a powder compact; sintering the compact; subjecting the sintered compact to solution treatment; and aging the solution-treated compact.
When a Sm2Co17-based alloy is subjected to high-temperature heat treatment for an extended period of time, the samarium undergoes evaporation on account of its very high vapor pressure, altering the composition of the magnets produced, which may lead to a deterioration in the magnetic properties, such as variable coercivities. On the other hand, low-temperature, short-duration heat treatment carried out to avoid samarium evaporation fails to provide a sufficient heat treatment effect, which leads to declines in the residual flux density and maximum energy product. Use of the alloy ingot according to the first aspect of the invention allows optimal heat treatment to be carried out in a short period of time, enabling the crystal grain size to be increased without unwanted changes in composition. Moreover, such Sm2Co17-based magnet alloys, when subsequently subjected to milling, molding of the milled powder in a magnetic field, sintering of the molded powder compact, solution treatment and aging treatment, can be used to produce Sm2Co17-based sintered magnets having excellent magnetic properties.
In a fourth aspect, the invention provides a method of manufacturing rare-earth permanent magnets, which method includes the steps of using a strip-casting process to form an alloy consisting essentially of 20 to 30 wt % of a rare-earth component R which is samarium alone or at least 50 wt % samarium in combination with at least one other rare-earth element, 10 to 45 wt % of iron, 1 to 10 wt % of copper and 0.5 to 5 wt % of zirconium, with the balance being cobalt and inadvertent impurities; heat-treating the strip-cast alloy in a non-oxidizing atmosphere at 1000 to 1300xc2x0 C. for 0.5 to 20 hours to form a rare-earth magnet alloy having an average grain size of 20 to 300 xcexcm; milling the rare-earth magnet alloy; compression-molding the milled alloy in a magnetic field to form a powder compact; sintering the compact; subjecting the sintered compact to solution treatment; and aging the solution-treated compact.
The foregoing method overcomes the deterioration in magnetic properties which is characteristic of sintered magnets obtained from conventional ingots cast in box-shaped molds, and is attributable in part to undesirable effects at the interior of the ingot such as coarsening of the structure and segregation of the composition owing to the formation of equiaxed crystals. Moreover, it avoids a problem normally associated with ingots having a microcrystalline structure that are produced by a single-roll strip casting process; namely, the formation of milled powder particles which are polycrystalline. Polycrystallinity is undesirable because it lowers the degree of orientation by the particles when the milled powder is pressed and shaped in a magnetic field, resulting in a low degree of orientation in the sintered magnet after heat treatment, which in turn lowers the residual flux density and the maximum energy product of the magnet. Hence, the rare-earth permanent magnet production method according to the fourth aspect of the invention can be used to produce Sm2Co17-based sintered magnets having excellent magnetic properties.
In a fifth aspect, the invention provides a method of manufacturing rare-earth sintered magnets, which method includes the steps of using a strip-casting process to form an alloy having the compositional formula:
R(Co(1-a-b-c)FeaCubZrc)z 
wherein R is samarium alone or at least 50 wt % samarium in combination with at least one other rare-earth element, and the letters a, b, c and z are positive numbers which satisfy the following conditions 0.1xe2x89xa6axe2x89xa60.35, 0.02xe2x89xa6b 0.08, 0.01xe2x89xa6cxe2x89xa60.05, and 7.0xe2x89xa6z9.0; heat-treating the strip-cast alloy at 1100 to 1250xc2x0 C. for 1 to 20 hours in a non-oxidizing atmosphere to form a rare-earth magnet alloy having a TbCu7-type crystal structure of at least 50 vol %; milling the rare-earth magnet alloy; compression-molding the milled alloy in a magnetic field to form a powder compact; sintering the compact; subjecting the sintered compact to solution treatment; and aging the solution-treated compact.
The foregoing method resolves the deterioration in magnetic properties which is characteristic of sintered magnets obtained from conventional ingots cast in box-shaped molds, and is attributable in part to undesirable effects at the interior of the ingot such as coarsening of the structure and to segregation of the composition owing to the formation of equiaxed crystals. Moreover, it eases the optimal temperature conditions for sintering and solution treatment which until now have had to be strictly controlled, thus enhancing productivity.
In a sixth aspect, the invention provides an anisotropic rare-earth sintered magnet which has been produced by milling a Sm2Co17-based permanent magnet alloy, followed by molding, sintering, solution treatment and aging treatment, the alloy consisting essentially of 20 to 30 wt % of a rare-earth component R which is samarium alone or at least 50 wt % samarium in combination with at least one other rare-earth element, 10 to 45 wt % of iron, 1 to 10 wt % of copper, 0.5 to 5 wt % of zirconium and 0.01 to 1.0 wt % of titanium, with the balance being cobalt and inadvertent impurities, which alloy has a TbCu7-type crystal structure content of at least 50 vol %. The magnet has a maximum energy product (BH)max of at least 25 MGOe. The alloy of which the magnet is made has an average crystal grain size of preferably 20 to 300 xcexcm.
In a seventh aspect, the invention provides a method of manufacturing an anisotropic rare-earth sintered magnet having a maximum energy product (BH)max of at least 25 MGOe, which method includes the steps of heat-treating a Sm2Co17-based permanent magnet alloy consisting essentially of 20 to 30 wt % of a rare-earth component R which is samarium alone or at least 50 wt % samarium in combination with at least one other rare-earth element, 10 to 45 wt % of iron, 1 to 10 wt % of copper, 0.5 to 5 wt % of zirconium and 0.01 to 1.0 wt % of titanium, with the balance being cobalt and inadvertent impurities, at 1100 to 1250xc2x0 C. for 0.5 to 20 hours to give the alloy a TbCu7-type crystal structure content of at least 50 vol %; milling the magnet alloy; molding the milled alloy to form a powder compact; sintering the compact; solution-treating the sintered compact; and carrying out aging treatment on the solution-treated compact.
The foregoing method overcomes the deterioration in magnetic properties which is characteristic of sintered magnets obtained from conventional ingots cast in box-shaped molds, and is attributable in part to undesirable effects at the interior of the ingot such as coarsening of the structure and segregation of the composition owing to the formation of equiaxed crystals. Moreover, it eases the optimal temperature conditions for sintering and solution treatment which until now have had to be strictly controlled, thus enhancing productivity. Additionally, by setting the average crystal grain size within a range of 20 to 300 xcexcm, milling does not result in the formation of a polycrystalline powder which would lower the degree of orientation during molding of the powder in a magnetic field, lower the degree of orientation in the sintered magnet following heat treatment, and ultimately lower the residual flux density and maximum energy product. Accordingly, Sm2Co17-based sintered magnets having excellent magnetic properties can be obtained.